Yevamot

[The comparison] might well be justified where the deceased brother married [first]  and the surviving brother married [his brother's wife's sister] afterwards,  for, in this case, since the prohibition of brother's wife was removed,  that of wife's sister  is also removed; but where the surviving brother had married [first] and the deceased brother had married subsequently, the prohibition of wife's sister was Surely in force first!  Furthermore, even where the deceased had married [first], [the comparison] would be justified in the case where the deceased had married and died, and the surviving brother had married afterwards so that [the widow] was eligible in the interval;  where, however, the deceased had married, and before he died his wife's sister was married by his surviving brother, [his widow] was never for a moment eligible for his brother! Does not 'Ulla admit that if the leper observed semen on the night preceding the eighth day  of his purification he must not project his hand into the Sanctuary on account of his thumb  because at the time he was eligible to bring the sacrifice [of the cleansed leper]  he was not free from uncleanness? But [this is really the explanation]: If ''aleha' was at all needed, [it was for such a case as] where the deceased brother had married [first] and died, and the surviving brother married [the widow's sister] subsequently. If you prefer I can say [that the reason  is because] it  might have been deduced by means of R. Jonah's analogy. For R. Jonah — others say, R. Huna son of R. Joshua — said: 'Scripture stated: For whosoever shall do any of these abominations shall be cut off,  all forbidden relatives were compared to a brother's wife';  [so in this case also it might have been said], as a brother's wife is permitted  so also are all other forbidden relatives permitted; hence the All Merciful has written, ''aleha'. Said R. Aha of Difti  to Rabina: Consider! All forbidden relatives  might be compared to a brother's wife  and might equally be compared to a wife's sister,  what reason do you see for comparing them to a wife's sister?  Compare them rather to a brother's wife!  — If you wish I might say: When a comparison may be made for increasing as well as for decreasing restrictions, that for increasing restrictions must be preferred. If you prefer, however, I might say: In the former cases  there are two prohibitions in the one as well as in the other,  and a double prohibition may justly be inferred from a double prohibition; in the latter case, however,  only one prohibition is involved,  and a double prohibition may not be inferred from a single one. Raba said: [That] a forbidden relative herself  [may not contract the levirate marriage] requires no Scriptural text to prove it, since no positive precept can supersede a prohibition which involves kareth; if a Scriptural text was at all needed it was for the purpose of forbidding a rival. And in the case of a forbidden relative is no Scriptural text required [to prohibit her levirate marriage]? Surely it was taught, 'Thus we are in a position to know the law concerning herself'!  — On account of her rival.  Was it not taught, however, 'Now we know the law concerning themselves'?  — On account of their rivals. Come and hear: Rabbi said: [Instead of] and take, [Scripture stated], and take her,  [and instead of] and perform the duty of a husband's brother [Scripture stated], and perform the duty of a husband's brother unto her,  in order to prohibit  [the levirate marriage of] forbidden relatives and their rivals!  — Read, 'To forbid [the levirate marriage of] the rivals of the forbidden relatives'. But two texts, surely, were mentioned;  was not one for the forbidden relative and the other for her rival? — No; both were for the rival, but one indicates prohibition  of a rival where the precept  is applicable, and the other indicates permission to marry the rival where the precept  is not applicable.  What is the reason? — [Because instead of] 'And perform the duty of a husband's brother' [Scripture stated] And perform the duty of a husband's brother UNTO HER, [which indicates that] only where levirate marriage is applicable is a rival forbidden  but where levirate marriage is not applicable  a rival is permitted.  R. Ashi said: [This  may] also be inferred from our Mishnah where it was stated, FIFTEEN [CATEGORIES OF] WOMEN EXEMPT THEIR RIVALS, but it was not stated, 'are exempt  and exempt [their rivals]'. This proves it. In what respect does the case of a forbidden relative differ  that it should require no text?  Obviously because no positive precept may supersede a prohibition which involves kareth. But then the case of a rival also should require no text,  since no positive precept may supersede a prohibition which involves kareth!  — Said R. Aha b. Bebai Mar to Rabina, Thus it has been stated in the name of Raba: In the case of a rival also no Scriptural text  was needed; if a text was needed at all